1. Field of the Invention
This invention relates generally to expandable intraluminal vascular grafts, generally referred to as stents, and more particularly concerns stents coated with a polymer component capable of carrying and releasing therapeutic drugs, and a method of incorporating therapeutic drugs into the polymer component of such stents.
2. Description of Related Art
Systemic administration of some therapeutic pharmaceutical drugs in order to provide an efficacious concentration at a localized area of interest can produce serious systemic side effects. Local administration of a therapeutic drug can be significantly more effective and produce fewer side effects than systemic delivery, particularly for anticoagulants used for preventing thrombosis of a coronary stent, and cytostatic agents applied for reducing postangioplasty proliferation of vascular tissue, which is a factor in restenosis after angioplasty.
There has thus been a need in modern medicine for techniques for local delivery of therapeutic drugs. In one technique, catheters have been used to deliver therapeutic drugs in a solution used to bathe the tissue for a short period of time, which is effective for administration of thrombolytic drugs, or to inject the drug solution into the tissue surrounding the area of interest. However, the therapeutic effect of drugs delivered by this method is generally relatively short, as the drugs are commonly easily eliminated from the delivery site.
Therapeutic drugs have been incorporated into relatively permanent structures for longer term delivery of the drugs at the site of interest. For example, extravascular wraps having a relatively large quantity of a drug in a bandage structure can be applied around the exterior of an artery. Although work in animals has shown that this technique is effective for local anticoagulation with heparin, it has limited practical utility in human beings, since an invasive operation is required to place the extravascular wrap at the site of interest.
In another technique, coatings of therapeutic drugs have been applied to stents in order to provide sustained delivery of the drugs at the site of interest. However, while coatings thick enough to provide a therapeutically effective amount of the drug can severely compromise the function of the stent, very thin coatings that do not impede the function of the stent, such as heparin coatings with a thickness of several microns, typically do not deliver a therapeutically effective amount of the drug.
It would be desirable to provide a method of incorporating a therapeutically effective amount of a therapeutic drug into an intravascular layer of polymeric material which can be applied to a relatively permanent intravascular device, such as a stent, that does not require an invasive operation for placement of the device, in a way that would not interfere with the functioning of the stent. The present invention meets these needs.